The present invention relates to material handling vehicles, and more particularly to a mechanism for adjusting the height of forks attached to a material handling vehicle.
Material handling vehicles, such as pallet trucks, typically includes a wheel mounted tractor upon which a fork assembly having a pair of load supporting forks is mounted for vertical movement between a raised and lowered position. Each fork is typically an elongated U-shaped structure including a pair of reinforced opposing walls which define a space in which a fork trail assembly is housed. The fork trail assembly is fixed to each fork proximal a fork distal end, and extends and retracts to support the fork distal end.
Each fork trail assembly typically includes a pivot arm pivotally fixed to the pivot shaft fixed to the fork. A ground engaging wheel is rotatably mounted to the pivot arm. The pivot arm is pivoted to maintain contact of the ground engaging wheel with a support surface as the fork is raised and lowered to support the distal end of the fork. The pivot arm is pivoted by a pull rod, or other link member, having one end pivotally fixed to the pivot arm and an opposing end pivotally linked to a bell crank. The bell crank is linked to the tractor, and pivots as the fork assembly moves between the raised and lowered positions to exert a force on the pull rod and pivot the pivot arm to extend or retract the ground engaging wheel. The slope of the forks (i.e. the height of the fork distal end relative to the height of the fork proximal end) is typically adjustable to compensate for load wheel wear, stacking of manufacturing tolerances, and pallet entry.
One method known in the art for adjusting the slope of the forks is to provide an adjustable length threaded pull rod to partially extend or retract the ground engaging wheel. Changing the length of the rod to partially extend or retract the ground engaging wheel vertically moves the fork distal end without changing the vertical position of the fork proximal end. Unfortunately, the threaded portions of the adjustable length rod are subject to stress concentration and cross section reduction which reduces the structural strength of the rod, and is thus a weak link prone to failure in the linkage connecting the tractor to the fork trail assembly.
In order to avoid the weaknesses in an adjustable length pull rod, a fixed length pull rod can be used. If a fixed length pull rod is used, however, in order to modify the length of the pull rod to adjust the fork slope, the pull rod must be cut to the desired length or an extension added, such as by welding. Unfortunately, modifying the length of a fixed length pull rod is labor intensive and degrades the serviceability of the truck in the field.
Both of the above solutions requires lifting the pallet truck off of the ground to gain access to the pull rod. This often requires removing the battery to reduce the weight of the truck prior to lifting. Moreover, changing the length of the pull rod only adjusts the height of the fork distal end and does not vertically move the fork proximal end. Vertically moving the fork proximal end is sometimes desirable to secure the necessary fork slope for pallet entry purposes. Therefore, a need exists for a fork assembly height adjustment mechanism which is simple, does not require modifying the length of the pull rod, and can adjust the height of the fork proximal end relative to the fork distal end.